Data communications by wireless LAN such as IEEE802.11 are now widely used, for instance, in personal computers (PCs), PC peripherals such as printers, hard disk drives and broadband routers, electronic apparatuses such as FAXs, refrigerators, standard television sets (SDTVs), high-definition television sets (HDTVs), digital cameras, digital video cameras and mobile phones, signal-transmitting means in automobiles and aircrafts, etc.
As the wireless LAN standard, IEEE802.11a uses an orthogonal frequency division multiplexing (OFDM) modulation system in a frequency band of 5 GHz, supporting high-speed data communications of maximum 54 Mbps. IEEE802.11b uses a direct sequence spread spectrum (DSSS) system in an industrial, scientific and medical (ISM) band of 2.4 GHz usable without a wireless license, supporting high-speed communications of 5.5 Mbps and 11 Mbps. IEEE802.11g uses the OFDM modulation system in a 2.4-GHz band like TFEE802.11b, supporting high-speed data communications of maximum 54 Mbps. WiMAX (IEEE 802.16-2004, IEEE 802.16e-2005, etc.) proposed as a high-speed wireless communications standard covering several kilometers of communications is expected as a technology for so-called last one mile of optical communications using three frequency bands of 2.5 GHz, 3.5 GHz and 5 GHz.
Important to high-frequency devices using pluralities of communications systems such as wireless LAN, WiMAX, etc. is how to separate transmitting and received signals in these communications systems. For example, WO 2006/003959 discloses a high-frequency circuit capable of performing diversity reception, comprising two dual-band antennas capable of transmitting and receiving signals for two communications systems (IEEE 802.11a and IEEE 802.11b) of different frequency bands, a high-frequency switch having four ports for switching connections to transmitting circuits and receiving circuits, a first multiplexer circuit disposed between one port of the high-frequency switch and the transmitting circuit, and a second multiplexer circuit disposed between the other port of the high-frequency switch and the receiving circuit.
Much attention is recently paid to wireless communications in MIMO (multiple-input, multiple-output) systems having excellent communications characteristics. The MIMO system requires that one communications system has pluralities of receiving terminals capable of simultaneously receiving signals independently. The term “MIMO” used herein includes slim (single-input, multiple-output). However, the high-frequency circuit described in WO 2006/003959 cannot handle a combination of multiband wireless communications and MIMO. Also, because wireless communications systems of a MIMO system have an increased number of circuit structures such as receiving terminals, etc. for one communications system, isolation is difficult between pluralities of communications systems, resulting in complicated circuit structures. Therefore, the use of the MIMO system in multiband wireless communications is extremely difficult. In the case of WiMAX handling particularly high transmission power, isolation among pluralities of communications systems is important to reduce transmission power loss.